Strange vacuum\idle...
Thread Starter
|
Senior User
Joined: Aug 2000
Posts: 205
Likes: 0
Strange vacuum\idle...
Hey guys, im having a wierd problem with my 84 f250 with, well ill call it a 460 to keep things simple since everything off the old 460 is on the new 429. anyway, i was doing some little tuning things on it and pulled a hose off the rear vacuum tee so i could put a vacuum gage on it. well the second i pulled the hose off, the rpm's jumped up a couple hundred...strange, because in all my experiences, loss of vacuum slows it down. before this, all it would do off the line is just barely break loose a tire, then i took it out for a spin with the hose off and it layed down a 20some foot strip of melted rubber, only, right when i stomped the gas, it hesitated for a split second before starting to roar. i put the hose back on because, if i remember right, it went up to another tee on the firewall that then ran hoses to the tranny, the cruise control, and the heater control stuff. any thoughts? im thinking it probably has something to do with the vacuum advance...its strange though, because when i adjust the timing to as high of an rpm as i can get it, it still goes higher when i pull the hose off. or could it be that its running way too rich and the extra air is compensating for it?
Senior User
Joined: Jan 2001
Posts: 326
Likes: 0
Strange vacuum\idle...
When you pulled the hose off the tree, you created a vac leak which cause's the intake charge to go lean. This has, in my experience, always caused the RPM's to increase. I don't know much about 429/460 motors, my experience is with 302/351W and 300I6 motors, but I would suggest that if you want the preformance you experienced with the vac leak:
-replace the hose on the tree
-reset the timing to where it should be
-then adjust the air bleed screws on the carb to lean out your intake charge
Just be careful how much you lean out the air/fuel mixture, too lean a charge can cause extermely high cylinder temperatures that can lead to serious engine damage, ie: stuck rings and piston meltdown.
RCC
-replace the hose on the tree
-reset the timing to where it should be
-then adjust the air bleed screws on the carb to lean out your intake charge
Just be careful how much you lean out the air/fuel mixture, too lean a charge can cause extermely high cylinder temperatures that can lead to serious engine damage, ie: stuck rings and piston meltdown.
RCC
Senior User
Joined: Jan 2001
Posts: 416
Likes: 0
Strange vacuum\idle...
The tee on the fire wall that goes to the tranny is the vaccuum for the tranny shift modulator. It will not shift properly without it Also with an open vaccuum you will not have sufficient vaccuum to operate the brakes properly ,if at all(no boost) This is a problem I just went trought this past weekend wih a friend truck. With an open line your idle will be incorrect and there will be too much air to fuel for the engine and the carb will not be able to compensate for the lean condition and piston meltdown could occur. This is why the engine bogs when you stomp on it because it is going realy lean before the fuel can catch up to the exessive air. Sure you will be able to light up your tire as I would expect a 460 to do anyway but you may also do a melt down too. Best check it out and correct the problem.
Thread Starter
|
Senior User
Joined: Aug 2000
Posts: 205
Likes: 0
Strange vacuum\idle...
ya, i didnt run it like that for longer than 2 minutes, i just felt the higher idle and took it out to see what the power felt like, then put the hose back on. and yes you make a good point, the big block shoudl be able to "light up a tire" anyway, which is telling me theres something definitely wrong, more than likely the fuel mixture being too rich. another thing ive noticed is that it always smells like its running too rich, and is getting horrible gas milage. ive tried adjusting the mixture screws, but since that only affects idle, it hasnt helped. i should probably just tear into the carb....
